Documentation

Introduction

Treadmill running is a widely used assessment of exercise performance in rodents. The involvement of multiple physiological systems, including cardiovascular, respiratory and metabolic parameters, makes this test a valid model of global performance. The test is amenable to diverse genetic backgrounds and manipulations, as well as to screening drug candidates to reduce fatigue. The treadmill test is a reliable, simple and high throughput assay that can be completed in a short time frame. In comparison to other fatigue tests, the treadmill does not rely on voluntary activity and enables the definition of objective and humane exhaustion criteria. Running time, distance traveled, maximum speed and number of shocks are the most common parameters to be analyzed.

For optimal testing, we recommend the use of a multi-lane system to allow for simultaneous testing of multiple animals. The treadmill should be connected to a source of aversive stimulation, which is typically a shock grid of adjustable intensity. The use of an automated system facilitates control of parameters and recording of results

Different protocols can be established in order to assess short-term and acute exercise or endurance performance. Moreover, within each category, increasing running speed and slope will enhance test stringency. Typically, after a training period, testing protocols entail gradual increases in running speed to let animals warm up and adjust their metabolic and cardiovascular parameters. Appropriate definition of termination criteria is essential. These can be based on time spent at the end of the lane (on or by the shock grid) and/or an increase in aversive stimulations.

Apparatus & Equipment

In order to perform the treadmill test, we recommend the use of an automated multi-lane system with adjustable speed, acceleration and slope. Lanes should be independent, covered and fit mice or rats of different sizes. Walls should be clear to facilitate observation.

An aversive stimulus should be used to force the animals to run. We recommend using a shock grid with adjustable intensity at the rear of the lanes. A light signal can be associated with the shocks to serve as unconditioned stimulus. Furthermore, the apparatus should have a platform where the animals can escape when exhausted.

We recommend the use of a computer connected to the treadmill to enable easy control of treadmill, shock grid and slope, and display treadmill parameters such as speed, distance traveled, time spent running and number of shocks received.

Protocol

Animals should be tagged to allow for rapid identification. It is critical that the investigators are blind to experimental conditions. If performing multiple runs on the same day (not simultaneous), the investigator should take care with study design to minimize variability. Specifically, consistency is key regarding time of day when the test is performed or time elapsed since the last session. Moreover, each session should include animals from all experimental groups.

Prior to start training the animals, the investigator should set the desired slope, which should remain constant during the entire experiment. Marcaletti et al. (2011) recommend 5° angle, whereas Dougherty et al. (2016) recommend 10°. Conversely, Nunomiya et al. (2016) did not use slope. Increasing the slope will make the test more stringent. In contrast, the experimenter can use a downhill setup to model exercise-elicited muscle damage (Marcaletti et al., 2011). Slight variations in electric shock frequency and intensity are found across different laboratories in training protocols. Dougherty et al. (2016) used 2 Hz, 1.22 mA (with each shock lasting 0.2 sec), whereas Marcaletti et al. (2011) used 0.2 mA for training and 0.3 mA for testing. The shock should be mild to avoid inducing major distress and delivered in pulsatile fashion.

Training

A few sessions of habituation to the setup and the task are required. Marcaletti et al. (2011) reported that, for C57BL6/J mice, 1 to 2 sessions are typically sufficient to yield less than 5 electrical stimulations. However, animal strain, sex and age can influence the extension of training. (Lightfoot et al., 2001, Lerman et al., 2002). Observation of the animal’s behavior is important in order to properly adjust the number of sessions. If an animal displays consistently poor training performance, it should be removed from the study.

Animals should be carefully placed into individual lanes. Before turning on the apparatus, the animals should be allowed 5-10 minutes to explore the new environment. Dougherty et al. (2016) suggest starting the shock grid before the treadmill and allow the animals enough time to explore its lane and/or to receive at least one shock. Regarding treadmill speed during training, Marcaletti et al. (2011) used 15 cm/sec for 5 min, whereas Dougherty et al. (2016) suggest starting with 1.5 to 3.0 m/min and then slowly increasing up to 10 m/min until stopping the session at 10 min. If the animal does not walk or walks toward the shock grid, the investigator should intervene and use a soft brush or tickle the tail. Number of aversive stimulations received should be recorded.

After completion of training, animals should be returned to their home cage and the apparatus should be cleaned with 70% ethanol.

Testing

We recommend allowing at least one day without contact with the treadmill before starting testing the animals. The investigator should define consistent termination criteria that, if met, the animal should be removed from the apparatus. In this regard, Dougherty, et al. (2016) suggest 5 consecutive seconds in the “fatigue zone”, which is the area within 1 body length of the shock grid. In addition, Marcaletti et al. (2011) recommend using the number of aversive stimulations, as it should substantially increase when the animal reaches its maximal capacity.

Animals should be placed on the belt immediately before starting the apparatus and the shock grid. The investigator should not intervene during testing to avoid introducing confounding variables. Treadmill settings for testing can be modified depending on study requirements. Dougherty et al. (2016) recommend setting speed to 12 m/min and gradually increase it up to 26 m/min over a 75-min session. Conversely, Marcaletti et al. (2011) suggest “power test” and “endurance test” settings: for “power test”, an initial speed of 15 cm/sec is recommended with 2cm/sec increases every minute for a maximum of 40 min. For “endurance test”, the same initial speed is suggested, but increases are 3 cm/sec every 12 min for a maximum duration of 4 hr. On the other hand, Nunomiya et al. (2016) set the apparatus at 10 m/min for the first 30 min and increased it by 2 m/min every 15 min. Anticipated duration and distance correlate with intensity of exercise. For C557BL/6 mice, “power” tests should take 20 min, while “endurance” tests should last 2 hr and mice are expected to run 2 km (Marcaletti et al., 2011).

At the end of the session or when all animals reach the termination criterion, apparatus should be cleaned as above. If retesting, Marcaletti et al. (2011) recommend a recovery period of 1 day after a “power test” and 7 days after an “endurance test”.

Running time, distance traveled, maximum speed reached by the animal and number of aversive stimulations should be recorded.

Modifications

The treadmill apparatus can be coupled to an open circuit calorimeter to allow assessment of maximal oxygen consumption (VO2max), carbon dioxide production (VCO2), and respiratory exchange ratio (Marcaletti et al., 2011). In this case, the software used to control the treadmill should also enable adjustment and display of variables related to the open-circuit calorimeter.

If testing animals more than 7 days after completing training, a pilot study is recommended to verify that they will perform during testing.

If desensitization of the animals is observed after repeated testing, the investigator should slightly increase the intensity of the shock. In alternative to electrical shocks, pulses of air or manual prodding of the animal can be used as aversive stimuli (Marcaletti et al., 2011).

Sample Data

Distance traveled (A) running time (B) and maximum speed (C) are the most commonly shown results when performing the treadmill test. Data can be shown as group averages on a single day (A and B) or as variation across test days (C, here representing days after administration of a drug candidate). Results are shown as mean ± standard error of the mean:

Strengths & Limitations

Strengths

The treadmill test is a high throughput assay that can be completed in a short time frame. Unlike voluntary wheel running activity, the treadmill test does not require individual housing and does not rely upon voluntary activity, which could introduce a confounding variable when analyzing fatigue (Dougherty et al., 2016). Moreover, this test allows for simultaneous testing of multiple animals and could be used to screen drug candidates to reduce fatigue. In comparison to fatigue tests involving swimming, the treadmill enables the definition of objective and humane exhaustion criteria.

Limitations

Genetic or pharmacological manipulations that affect locomotion, metabolism or behavior may severely disrupt performance. In addition, Dougherty et al. (2016) observed a decrease in overall compliance after repeatedly testing the same mice. Testing of very young or small animals should be avoided to prevent risk of injury. Also, strong reaction to the shock may cause the animals to spend less time walking on the treadmill and to escape.

Summary & Key Points

The treadmill test is the gold standard to assess maximal exercise performance in rodents.

The animal is required to run on a belt with electrical shocks being used as aversive stimulation at the end of the lane. Slope, speed, session duration and shock intensity can be adjusted.

The use of an automated system enables easy and consistent adjustment of settings and recording of results.

This test is a high throughput assay that enables the screening of drug candidates to reduce fatigue in rodents. It can be completed in a short time frame and offers the possibility to adjust test settings depending on study specificities. In comparison to other exercise performance protocols, the treadmill test does not rely upon voluntary activity and allows for the definition of objective and humane exhaustion criteria.

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